Intermittent DC bias charge roll AC cleaning cycle

ABSTRACT

An apparatus for applying an electrical charge to a charge retentive surface, wherein a bias charge roll member is situated proximately to a surface to be charged such as, a photoreceptor. The bias contact roll member is supplied with an electrical bias including an oscillating voltage signal having, in one mode of operation, a clipped AC input voltage and in a second mode, an unclipped AC input voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly-assigned copending U.S. patent applicationSer. No. 10/319,172, filed herewith, entitled INTERMITTENT DC BIASCHARGE ROLL WITH DC OFFSET VOLTAGE, by Facci, et al, the disclosure(s)of which are incorporated herein.”

FIELD OF THE INVENTION

The present invention relates generally to a roller apparatus forgenerating a substantially uniform charge on a surface, and, moreparticularly, concerns a biased roll charging apparatus having, in onemode of operation, a clipped AC input voltage, primarily for use inelectrostatographic applications and in a second mode, an unclipped ACinput voltage.

BACKGROUND AND SUMMARY

When used to charge an imaging member, a roller used to create a chargeon another surface or substrate is commonly referred to as bias chargeroll (“BCR”). When used to charge a substrate to enable transfer of adeveloped image from an imaging member to a substrate member, a rollerused to create such bias charging is commonly referred to as a biastransfer roll (“BTR”). Although both may differ in details particular totheir applications, both represent illustrative embodiments of thepresent invention.

Generally, the process of electrostatographic reproduction is initiatedby substantially uniformly charging a photoreceptive member, followed byexposing a light image of an original document thereon. Exposing thecharged photoreceptive member to a light image discharges aphotoconductive surface layer in areas corresponding to non-image areasin the original document, while maintaining the charge on image areasfor creating an electrostatic latent image of the original document onthe photoreceptive member. This latent image is subsequently developedinto a visible image by a process in which a charged developing materialis deposited onto the photoconductive surface layer, such that thedeveloping material is attracted to the charged image areas on thephotoreceptive member. Thereafter, the developing material istransferred from the photoreceptive member to a copy sheet or some otherimage support substrate to which the image may be permanently affixedfor producing a reproduction of the original document. In a final stepin the process, the photoconductive surface layer of the photoreceptivemember is cleaned to remove any residual developing material therefrom,in preparation for successive imaging cycles.

The above described electrostatographic reproduction process is wellknown and is useful for both digital copying and printing as well as forlight lens copying from an original. In many of these applications, theprocess described above operates to form a latent image on an imagingmember by discharge of the charge in locations in which light from alens, laser, or LED discharges a charge. Such printing processestypically develop toner on the discharged area, known as DAD, or “writeblack” systems. Light lens generated image systems typically developtoner on the charged areas, known as CAD, or “write white” systems. Theembodiments of the present invention apply to both DAD and CAD systems.

With respect to BCR applications, those skilled in the art recognizethat various devices and apparatus have been proposed for creating auniform electrostatic charge or charge potential on a photoconductivesurface prior to the formation of the latent image thereon. Generally,corona generating devices are utilized to apply a charge to thephotoreceptive member. In a typical device, a suspended electrode, orso-called coronode, comprising a thin conductive wire is partiallysurrounded by a conductive shield with the device being situated inclose proximity to the photoconductive surface. The coronode iselectrically biased to a high voltage potential, causing ionization ofsurrounding air which results in the deposit of an electrical charge onan adjacent surface, namely the photoconductive surface of thephotoreceptive member. Corona generating devices are well known, asdescribed, for example, in U.S. Pat. No. 2,836,725, to R. G. Vyverberg,among numerous other patents and publications. In the referencedVyverberg patent, the coronode is provided with a DC voltage, while theconductive shield is usually electrically grounded and thephotoconductive surface to be charged is mounted on a groundedsubstrate, spaced from the coronode opposite the shield. Alternatively,the corona device may be biased in a manner taught in U.S. Pat. No.2,879,395, wherein the flow of ions from the electrode to thephotoconductive surface is regulated by an AC corona generatingpotential applied to the conductive wire electrode and a DC potentialapplied to the conductive shield partially surrounding the electrode.The DC potential allows the charge rate to be adjusted, making thisbiasing system ideal for self-regulating systems. Various other coronagenerating biasing arrangements are known in the art and will not bediscussed in great detail herein.

Several problems have historically been associated with coronagenerating devices. One problem includes the use of very high voltages(3000-8000 V), requiring the use of special insulation, inordinatemaintenance of corotron wires, low charging efficiency, the need forerase lamps and lamp shields and the like, arcing caused bynon-uniformities between the coronode and the surface being charged,vibration and sagging of corona generating wires, contamination ofcorona wires, and, in general, inconsistent charging performance due tothe effects of humidity and airborne chemical contaminants on the coronagenerating device. More importantly, corotron devices generate ozone,resulting in well-documented health and environmental hazards. Coronacharging devices also generate oxides of nitrogen which eventuallydesorb from the corotron and oxidize various machine components,including the photoreceptor, resulting in an adverse effect on thequality of the final output print produced thereby.

As an alternative to corona generating devices used in charging systems,roll charging systems such as, BCR's and BTR's have been developed andincorporated into various machine environments with limited success. BCRcharging systems are exemplified by U.S. Pat. No. 2,912,586, to R. W.Gundlach; U.S. Pat. No. 3,043,684, to E. F. Mayer; U.S. Pat. No.3,398,336, to R. W. Martel et al.; U.S. Pat. No. 3,684,364, to F. W.Schmidlin; and U.S. Pat. No. 3,702,482, to Dolcimascolo et al., amongothers, wherein an electrically biased charging roller is placed incontact with the surface to be charged, e.g. the photoreceptive member.Also relevant is U.S. Pat. No. 5,412,455, to Ono et al. wherein acharging device includes: a member to be charged; a charging memberconnectable to the member to be charged; a power source for supplying anoscillating voltage to the charging member; and a constant voltageelement connected electrically in parallel with the power source forgenerating the oscillating voltage. Also, U.S. Pat. No. 5,463,450, toInoue et al. discloses a charging apparatus for electrically charging amember to be charged including a charging member contactable to themember to be charged. The member to be charged includes a core and avoltage source for applying an oscillating voltage between the member tobe charged and the charging member, wherein the frequency of theoscillating voltage satisfies a predetermined condition. Each of theseis hereby incorporated by reference in their entirety.

In BTR charging systems, DC voltage is typically used. DC voltageattracts dirt, however, especially toner in spaces void of printingsubstrates, such spaces comprising inter-document zones, areas exposedwhen printing on less-than-full-width printing media, and similar areasin which the BTR is directly exposed to the charge carrying member orintermediate transfer member. Paper debris is also another contaminantof BTR systems. In response, conventional BTR apparatus require brushesto remove dirt and debris. Such brushes, however, add cost andcomplexity, occupy valuable space, and require maintenance when cloggedor filled with dirt.

In BCR charging systems, a charging member in the form of a roller iscontacted with the surface of the photoreceptive member or other memberto be charged, and an oscillating input voltage, typically a DC biasedAC voltage signal, is applied to the roller to generate an oscillatingelectric field for applying a charge potential of a given polarity, tothe photoreceptive member where the DC offset defines the polarity ofthe charge applied. Although the input voltage may be comprised solelyof a DC component, an oscillating voltage such as, an AC voltage signalhaving a DC voltage signal superimposed thereon has been found to bepreferable with respect to charge uniformity. See, e.g., U.S. Pat. No.4,851,960 to Nakamura et al which teaches that peak-to-peak inputvoltage, Vp-p, for DC-biased AC wave form should be twice the chargestarting voltage for the photoreceptor or other charge receptor in thesystem being charged.

The absence of charge uniformity tends to manifest itself in the form ofperiodic stripes or so-called strobing corresponding to the variation incharge potential on the photoconductive surface. This strobing effectcauses variations in toner attraction during development and oftenresults in significant image quality degradation. However, anoscillating input voltage contributes both positive and negativepolarity charge to the photoconductive surface. This results in acharging system that requires relatively high charging currents which,in turn, has a negative effect on the functional life of thephotoreceptive member. Thus, a significant disadvantage of most biasedroll charging systems is the resulting rapid wear of the photoconductivesurface caused by the electrical discharge from the bias charge rollduring the charging process. A related cause for rapid wear appears tobe chemical degradation of organic and other complex molecules coupledwith repetitive wiping or scraping of the photoreceptor layers bycleaning blades or other cleaning members.

One partial solution to the above problems is found in U.S. Pat. No.5,613,173, issued to Kunzmann et al., hereby incorporated by referencein its entirety. In Kunzmann, a BCR apparatus is disclosed havingclipped AC input voltage to reduce the phenomenon of strobing while alsoreducing photoreceptor wear caused by the electrical discharge from thebias charge roll during the charging process. The clipping of the ACoscillating voltage removes one polarity from the input signal, therebysupplying a single polarity to the photoreceptor or other charged memberand, as a result, enabling sufficient charging at lower voltages appliedto the charged surface. Such lower voltages extend photoreceptor life,in part by reducing electrically induced chemical damage.

Although the solution of Kunzmann improves photoreceptor wear,elimination of an AC oscillating voltage leaves only DC voltage, andDC-only voltage attracts dirt such as, toner particles, dust, and paperdebris. One solution is to add cleaning apparatus such as, brushes andcleaning blades. These, however, add cost, complexity, and maintenanceissues. What is needed is a system that retains the improved durabilityadvantages of a single polarity waveform as well as the cleaningadvantages of a multiple polarity waveforms.

In accordance with one embodiment of the present invention, an apparatusfor applying an electrical charge to a member to be charged is provided,said charging apparatus comprising: a power supply for supplying anoscillating voltage signal; a charge roll member situated in proximityto a surface of the member to be charged; and a switch for switchingbetween a plurality of modes wherein: (a) in a first mode, an electricalbias is applied from the power supply to the charge roll member, theelectrical bias including a single polarity input drive voltage to saidcharge roll member; and (b) in a second mode, an electrical bias isapplied from the power supply to the charge roll member, the electricalbias including an oscillating voltage signal containing multiplepolarity components in order to supply oscillating polarity input drivevoltage to the charge roll member.

In accordance with another embodiment of the invention, anelectrophotographic imaging system is provided, said imaging systemcomprising: an apparatus for applying an electrical charge to a memberto be charged, said charging apparatus comprising: a power supply forsupplying an oscillating voltage signal; a charge roll member situatedin proximity to a surface of the member to be charged; and a switch forswitching between a plurality of modes wherein: (a) in a first mode, anelectrical bias is applied from the power supply to the charge rollmember, the electrical bias including a single polarity input drivevoltage to said charge roll member; and (b) in a second mode, anelectrical bias is applied from the power supply to the charge rollmember, the electrical bias including an oscillating voltage signalcontaining multiple polarity components in order to supply oscillatingpolarity input drive voltage to the charge roll member.

In accordance with another embodiment of the invention, a process forapplying an electrical charge to a member to be charged is presented,said process comprising: supplying an oscillating voltage signal from apower supply; positioning a charge roll member proximately to a surfaceof the member to be charged; and selecting between a plurality of modes,wherein: in the first mode, an electrical bias is applied from the powersupply to the charge roll member, the electrical bias including a singlepolarity input drive voltage to said charge roll member; and in a secondmode, an electrical bias is applied from the power supply to the chargeroll member, the electrical bias including an oscillating voltage signalcontaining multiple polarity components in order to supply oscillatingpolarity input drive voltage to the charge roll member.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will become apparentfrom the following description in conjunction with the accompanyingdrawings in which:

FIG. 1 is a partial schematic view of a biased roll charging system inaccordance with the prior art and showing the electrostatic operation ofthe system;

FIG. 2 is a graphical representation of the surface potentialdifferential that can be achieved by the bias roll charging system ofthe present invention [strictly speaking, this figure deals with theKunsmann et al patent, and the present invention makes sure that the BCRstays clean] relative to a conventional bias charge roll charging systemusing a non-clipped oscillating input voltage signal; and

FIG. 3 is a partial schematic view of a biased roll charging system inaccordance with one embodiment of the present invention;

FIG. 4 is partial schematic view of a biased roll charging system inaccordance with another embodiment of the present invention.

DESCRIPTION

For a general understanding of the present invention, reference is madeto the drawings. In the drawings, like reference numerals have been usedthroughout to designate identical elements.

It will be recognized, that while the present invention describes acharging system for a typical BCR used in an electrostatographicprinter, embodiments of the present invention are equally well suitedfor use in a wide variety of other electrostatographic-type processingmachines, in BTR applications, and in other applications in whichuniform charges are to be placed upon moving surfaces. The disclosedinvention is not limited in its application to the particular embodimentor embodiments shown herein. In particular, it should be noted that thecharging apparatus of the present invention, described with reference toan exemplary charging system, may also be used in a transfer, detack, orcleaning subsystem of a typical electrostatographic apparatus since suchsubsystems may also require the use of a charging device. In addition,it will be recognized that the disclosed biased roll charging system mayhave equal application for applying an electrical charge to a memberother than a photoreceptor and/or in environments outside the realm ofelectrostatographic printing.

Referring initially to FIG. 1, one embodiment of a biased roll chargingsystem is shown in the context of an exemplary electrostatographicreproducing apparatus, employing a drum 12 including a photoconductivesurface 35 deposited on an electrically grounded conductive substrate38. A motor (not shown) engages with drum 12 for rotating the drum 12 toadvance successive portions of photoconductive surface 35 throughvarious processing stations disposed about the path of movement thereof,as is well known in the art. Initially, a portion of drum 10 passesthrough a charging station where a charging device in accordance withthe present invention, indicated generally by reference numeral 10,charges the photoconductive surface on drum 12 to a relatively high,substantially uniform potential.

Referring now, more particularly, to the bias roll charging system 10, aconductive roll member 14 is provided in contacting engagement with thephotoreceptor member 12. The conductive roll member 14 is axiallysupported on a conductive core or shaft 20, situated transverse to thedirection of relative movement of the photoreceptor member 12. In oneembodiment, the roll member 14 is provided in the form of a deformable,elongated roller supported for rotation about an axis 16 and ispreferably comprised of a polymer material such as, for example,neoprene, E.P.D.M. rubber, Hypalon® rubber, nitrile rubber, polyurethanerubber (polyester type), polyurethane rubber (polyether type), siliconerubber, Viton®/Fluorel® rubber, epichlorohydrin rubber, or other similarmaterials having a DC volume resistivity in the range of 10³ to 10⁷ohm-cm after suitable compounding with carbon particles, graphite orother conductive additives. These materials are chosen for thecharacteristic of providing a deformable structure while in closeproximity or contact with the photoreceptor member, as well aswearability, manufacturability and economy. The deformability of theroller member 14 is important to provide a nip having a substantiallymeasurable width while being engaged with the photoreceptor 12.

A high voltage power supply 22 is connected to roll member 14 via shaft20 for supplying an oscillating input drive voltage to the roll member14. While it is possible to use a standard line voltage, other voltagelevels or voltage signal frequencies may be desirable in accordance withother limiting factors dependent on individual machine design such as,the desired charge level to be induced on the photoreceptor or the speedof imaging operations desired. The oscillating input voltage and circuitconnecting the power supply 22 to shaft 20 is discussed in greaterdetail below.

With particular regard to biased roll charging, a suitablephotoreceptive member 12 has the property of injecting a single sign ofmobile carriers from a charge generating layer into a charge transportlayer such that, a surface charge potential having only a single chargepolarity is generated on the surface of the photoreceptor member,irrespective of the inducing voltage signal applied to roll member 14.With reference to FIG. 1, the photoreceptive member 12 generallyincludes a conductive substrate 38, such as, an aluminum sheet connectedto a ground potential 37, a charge generating layer 30, a chargetransport layer 32 comprising a photoconductive insulator such as,selenium or any of a variety of organic compositions, and a overcoating34, forming the outer surface 35 of the photoreceptor member.

The charging operation involves the application of the AC voltage signalfrom the bias charging system 10 to the photoconductive surface ofphotoreceptor 12, which creates a voltage potential across thephotoreceptor to ground 37. Charge carriers from the charge generatinglayer 30 migrate into the bulk of the charge transport layer 32 to theupper surface 36 of the photoconductive material, where the charge willbe trapped. When the AC voltage signal from voltage source 22 is of anegative polarity, as indicated by the minus signs (−) along thelowermost portion of roller member 14, in contact with the outer surface35 of photoreceptor member 12, a positive charge indicated by plus signs(+) is induced near the upper surface 36 of the photoconductive materiallayer, suitable for charging the photoreceptor member in preparation forimaging. A thin dielectric overcoating 34 is desirable on either theroller member 14 or the photoreceptor 12 for a variety of reasons,including protection of the surfaces of roller member 14 orphotoreceptor 12, or for a current limiting action which may allow theuse of low resistivity rollers, or for photoreceptor or roll membersurface property control, and especially because the use of anovercoating allows operation of the device below typical coronathresholds, and so avoids strobing due to exit corona, as will bediscussed. In the embodiment shown in the drawings, overcoating 34 isprovided on the upper surface of the photoreceptor. Alternatively, anovercoating may be provided on the outer surface of bias roll member 14for the same effect.

Strobing (i.e. successive areas of varying voltage characteristics), hasat least two causes. It can be caused by inducing a charge on a firstphotoreceptor surface portion by providing roller member 14 in contactwith that portion during a period of the AC voltage signal passingthrough a selected polarity, while in a succeeding photoreceptor surfaceportion, inducing no charge because the AC voltage signal is passingthrough a period of non-selected polarity while roller member 14 is incontact with that portion of the photoreceptor surface. Accordingly, inorder to provide a uniform charge on the photoreceptor surface, eachincremental portion of the photoreceptor member surface must becontacted during a period of charging, or a period wherein the polarityof the driving voltage is of the selected polarity for charging. Thus, agiven area of the rubber roller 14, the nip, should be maintained incontact with any selected surface portion for a period greater than theperiod of the driving voltage frequency. Varying nip widths may beprovided by varying the materials used for the roller. In most cases,the allowable relative speed of the bias roller and the photoreceptorsurface is varied in compensation for the varied nip width to preventstrobing. It will, of course, be appreciated that the time required forcharging a photoreceptor to a given voltage level depends on the physicsof the charge transfer process. In other words, charging for apredetermined period is sufficient to charge the photoreceptor to adesired voltage level.

Strobing may also occur if the combination of induced and appliedcharges causes the field in the exit portion of the nip to exceed thetypical corona threshold. That is, in the area of the exit nip, airbreakdown may occur, resulting in deposit of surface charges on theroller and the photoreceptor. The amount of surface charge will bemodulated by the AC applied voltage. If this occurs, strobing may beeliminated by making the overcoating thicker or reducing the peakapplied voltage.

U.S. Pat. No. 5,613,173, issued to Kunzmann and discussed above,discusses the problems resulting from using a simple DC offset ACwaveform from power supply 22 to shaft 20. Specifically, the use of asimple AC waveform contributes both positive and negative charge to thephotoreceptor member. Since the photoreceptive member 12 has theproperty of injecting only a single sign of mobile carriers from acharge generating layer to induce the generation of only a single chargepolarity, a relatively high DC offset bias charge roll current isrequired in order to create a current with only one polarity. The highDC bias current, however, results in degradation and rapid wear of thephotoreceptor charge transport layer due to the electrical discharge ofthe bias charge roller as the photoreceptor member is being charged. Thesolution in Kunzmann is to clip, or rectify, the AC current, therebyproviding a single polarity oscillating input drive voltage supplied tothe bias charge roller. This approach allows a reduced total appliedvoltage to the bias roll system without limiting the resulting surfacecharge potential and its uniformity.

One specific embodiment described in Kunzmann is incorporated within theprior art circuit shown in FIG. 1. In this embodiment, a simplediode/resistor circuit 26, 28 is coupled to the high voltage powersupply 22 for eliminating the positive component of the DC offset ACwaveform provided by power supply 22 without the need for a DC offsetsignal. This diode/resistor circuit acts as a rectifier circuit foreliminating or clipping the positive component of the oscillating ACvoltage signal. As explained in Kunzmann, an exemplary embodiment in theart prior to Kunzmann comprises a bias charge roll input drive voltagehaving a peak-to-peak voltage of 1.6 kilovolts with a DC offset of minus350 volts at a frequency of 400 hertz. Such an input drive signal willresult in 450 volts of positive charge and 1150 volts of negativecharge. The resulting photoreceptor surface potential approximates minus330 volts. By clipping the positive component of this typical AC inputwaveform, aggregate current flow to the surface of the photoreceptor canbe reduced while maintaining required voltage levels. Such decreasedcurrent flow decreases the degradation and wear of the charge transportlayer of photoreceptor member 12.

Turning now to FIG. 2, copied from Kunzmann, the surface potential onthe photoreceptor is graphed as a function of both a clipped andunclipped AC input voltage Vp-p. As shown, the surface potential can beincreased over conventional AC waveforms in relation to an increase inthe peak-to-peak input voltage. In a conventional bias charge rollcharging system using a non-clipped oscillating input voltage signal,the surface potential generated on the photoreceptor tends to level off(at approximately 350 volts in FIG. 2) notwithstanding the continuedincrease in peak-to-peak input voltage. By contrast, in accordance withthe invention in Kunzmann, the surface potential generated by a biascharge roll charging system using a clipped oscillating input voltagesignal continues to increase as a function of the peak-to-peak inputvoltage, such that the leveling off characteristic described above withrespect to a non-clipped oscillating input voltage signal is eliminated.In this manner, an increased surface potential can be generated on thephotoreceptor with a reduced current flow into the photoreceptor whencompared to a conventional bias charge roll charging system using anon-clipped oscillating input voltage signal.

While the bias charge system taught by Kunzmann succeeds in loweringdamaging high input voltages and resulting highly charged corona aroundthe bias charging system, it exacerbates the need to clean the biascharging system. Specifically, DC-only bias charge systems attract andmaintain lint, dirt, toner, and paper debris. The same is also true withrespect to DC-biased AC bias charge systems in which the DC-bias resultsin a charge of constant polarity. The result is that in either thesystem proposed by Kunzmann or in a constant polarity DC-biased chargingsystem, additional cleaning systems are required. These cleaning systemstypically comprise brushes or blades that scrape across the surface ofthe bias charge roller and clean the roller either entirely throughmechanical brushing or scraping or by a combination of mechanicalbrushing/scraping and attraction of dirt by carrying a charge ofopposite polarity from the bias charge roller itself. Regardless of thetype of cleaning system, the added components add cost, complexity, andrequire additional space within a printer compartment. Worse, mechanicalbrushing or scraping inevitably wears the surface of the bias chargeroller itself, thereby shortening its useful life and causing itselectrical charging characteristics to drift over time.

Accordingly, one embodiment of the present invention comprises a systemthat combines the cleaning advantages of (i) conventional AC only ormulti-polarity DC-biased AC wave form charging systems with (ii) thelower voltage advantages of rectified oscillating DC-only wave forms.Turning now to FIG. 3, one embodiment of a bias charging system of thepresent invention is shown. Most features are identical to the prior artbias charging system in FIG. 1 and are labeled accordingly. The circuitconnecting high voltage power supply to shaft 20, however, differs.Specifically, switch 90 is interposed between high voltage power supply22 and rectifier 26. In one mode, switch 90 completes the circuitthrough rectifier 26 during normal imaging cycles. This results in allof the advantages set forth in Kunzmann, discussed above. In a secondmode activated during system warm-up and shutdown cycles and at leastperiodically during non-imaging periods, switch 90 circumvents rectifier26 to deliver conventional multi-polarity AC or DC-biased AC current tobias charge system 10. The result is a cleaning cycle during non-imagingperiods. Such cleaning cycle is accomplished without any extra cleaningapparatus other than switch 90. Brushes and blades that add complexityand that can scrape and scratch the bias charge roller can beeliminated. This embodiment of the present invention saves cost andcomplexity while increasing reliability.

An alternative embodiment of the present invention incorporates atwo-level DC-bias capability into high voltage power supply 22. Duringthe cleaning mode, the DC bias may be set to zero to avoid charging thephotoreceptor. During imaging periods where the AC signal is clipped, aDC bias of desired intensity may be supplied from the high voltage powersupply 22.

Those skilled in the art recognize that switch 90 can comprise any of alarge number of switching mechanisms commonly used in office-typeequipment. Examples include conventional single pole, single throwswitches, solid state switches, and solid state or electromechanicalrelays. In one embodiment, switch 90 operates in its first DC-only modeonly during imaging cycles. At all other times, switch 90 operates inits multi-polarity AC waveform mode. The system controller (not shown)identifies whether the system is in its imaging mode or non-imaging modeand issues software commands directing switch 90 to open and closecircuits in order to complete the circuit to either the DC-only mode orthe AC waveform mode.

In another embodiment, switch 90 opens the circuit to AC waveformcurrent during timed periods triggered by certain machine events suchas, system warm-up, shut-down, end of an imaging sequence, or systemidleness for a specified period. The overall duty cycle for the ACcleaning mode should generally be from 5% to 40% of the time duringwhich photoreceptor 12 is in motion in order to achieve acceptablecleaning of charge roller 10 and acceptable life of photoreceptor 12.Generally, the lower the percentage of duty cycle, the better, and aduty cycle less than 20% is preferred. Among other factors influencingthe required duty cycle are the amount of toner developed as backgroundoutside of the image area of the photoreceptor and the extent to whichan untransferred and uncleaned residual toner adheres to the surface ofthe BCR. During the period in which the cleaning mode is active, thephotoreceptor or other charge receptor should travel past the BCR atleast the distance of one rotation of the BCR.

In those instances when the cleaning mode is activated for aninsufficient portion of the duty cycle, a pause in the imaging mode anda switch to the cleaning mode for one rotation of BCR 10 can be used toensure adequate cleaning of the BCR. Such an insufficient proportion ofduty cycle might occur if the system is producing long running jobs andis continuously operating in the imaging mode. Alternatively to aphotoreceptor rotation, the proportion of duty cycle devoted to thecleaning mode can be increased by introducing the cleaning mode duringinter-document gaps if such gaps between the pitches are sufficientlylarge.

As taught in Nakamura et al., U.S. Pat. No. 4,851,960, uniform andadequate charging of photoreceptor 12 will occur if peak-to-peak ACvoltage, V_(p-p), is at least twice the charge starting voltage of thephotoreceptor. It is well known to those skilled in the art thatadequate cleaning will also occur if V_(p-p) is twice the chargestarting voltage of photoreceptor 12. Depending upon the nature ofparticles adhering to the BCR, V_(p-p) of 1.5 times the charge startingvoltage of the photoreceptor or other charge receptor in the systemappears adequate. Of course, the charge starting voltage differs for thewide variety of charge receptors that various systems utilize.

In one embodiment shown in FIG. 4, during the cleaning cycle,development of latent images upon the charge receptor 12 is suppressedin order to avoid pulling matter ejected from the BCR onto the chargereceptor or into the development apparatus. Such suppression can occurby disengaging operation of development apparatus 50 during cleaning.Alternatively, the bias potential within the development field emittedby the development apparatus can be adjusted during cleaning such thatV_(bias) minus the charge potential of the charge receptor is sufficientto suppress toner development but insufficient to promote reversedevelopment from the photoreceptor to the development unit. Forrepresentative organic photoreceptors, a typical range would be from 50Vto 200V. Within such a range, attraction of particles ejected from theBCR to the development unit is suppressed.

In another embodiment, the cleaning apparatus of charge receptor 12 isengaged during the BCR cleaning cycle. Such cleaning apparatus maycommonly comprise cleaning blades, ‘fur’ or insulative brushes, orelectrostatic brushes. In such an embodiment, the charge receptor 12 isrotated at least sufficiently for the region of the charge receptorexposed to BCR debris to sweep through the cleaning apparatus.

In yet another embodiment, V_(p-p) is increased during the AC cleaningmode. Such increase in voltage during cleaning is designed to increasethe cleaning effect upon dirt, paper, and other matter to be expelledfrom the BCR. One simple embodiment that accomplishes this increasedvoltage during the AC cleaning mode is shown in FIG. 4. Here, resister91 is inserted into the circuit with rectifier 26. One skilled in theart may elect any number of other methods for applying a greater voltageduring the AC cleaning mode, including utilizing a high voltage powersupply 22 with a two-level bias capability.

Another embodiment is applicable to the extent that the applied ACvoltage during the cleaning mode induces strobing due to variation incharge potential by the greater voltage AC waveform. Such strobing isparticularly possible if the AC V_(p-p) is increased in order toincrease the cleaning effect of the AC signal during the cleaning mode.In response, a complete rotation of charge receptor 12 in the DC-onlymode enables all portions of the charge receptor to sweep through theDC-only corona before imaging. Alternatively, the photoreceptor may beslowed during cleaning. Another alternative is to use an erase lamp onthe photoreceptor.

Each of the above embodiments may be applicable to BTR's as well asBCR's. In the case of BTR's, however, an AC cleaning mode may not besufficient to remove all dirt and debris. Supplemental cleaning meanssuch as, brushes may therefore be necessary. Even when such supplementalcleaning means are required, the AC cleaning mode removes much of thedirt and debris, thereby increasing the expected life of thesupplemental cleaning means and decreasing maintenance cost and efforts.

In review, the foregoing description discloses an apparatus for applyingan electrical charge to a photoreceptor wherein a bias contact rollmember is situated in contact with a surface of member to be chargedsuch as, a photoreceptor. The bias contact roll member is supplied withan electrical bias having at least two operating modes. In a first mode,an oscillating voltage signal is clipped to remove a predeterminedpolarity component thereof. In a second mode, the oscillating voltagesignal is not clipped, and a signal comprising multiple polarities isapplied to the contact roll member. In this manner, the improveddurability advantages of a clipped single polarity waveform can berealized as well as the cleaning advantages of a multiple polaritywaveform.

It is, therefore, apparent that there has been provided, in accordancewith the present invention, a roll charging device that fully satisfiesthe aims and advantages set forth hereinabove. While particularembodiments have been described, alternatives, modifications,variations, improvements, and substantial equivalents that are or may bepresently unforeseen may arise to applicants or others skilled in theart. Accordingly, the appended claims as filed and as they may beamended, are intended to embrace all such alternatives, modificationsvariations, improvements, and substantial equivalents.

1. An apparatus for applying an electrical charge to a member to becharged, comprising: a power supply for supplying an oscillating voltagesignal; a charge roll member situated in proximity to a surface of themember to be charged; and a switch for switching between a plurality ofmodes wherein: a) in a first mode, an electrical bias is applied fromthe power supply to the charge roll member, the electrical biasincluding a single polarity input drive voltage to said charge rollmember, said voltage signal comprising an oscillating voltage signalthat is clipped to remove a selected polarity; and b) in a second mode,an electrical bias is applied from the power supply to the charge rollmember, the electrical bias including an oscillating voltage signalcontaining multiple polarity components in order to supply oscillatingpolarity input drive voltage to the charge roll member.
 2. The apparatusof claim 1, wherein the charge roll member is a bias transfer roller. 3.The apparatus of claim 1, wherein in the first mode, the power supplysupplies an oscillating polarity component combined with a DC offsetcomponent in order to apply an electrical bias of a single polarity. 4.The apparatus of claim 1, wherein the second mode is selected duringnon-imaging periods.
 5. The apparatus at claim 1, wherein the secondmode is selected during system warm-up cycles.
 6. The apparatus of claim1, wherein the second mode is selected on during shutdown procedures. 7.The apparatus of claim 1, wherein the peak-to-peak oscillating polaritydrive voltage during a period in which the second mode is selectedexceeds twice the charge starting voltage of the member to be charged.8. The apparatus of claim 1, wherein the peak-to-peak oscillatingvoltage during a period in which the second mode is selected exceeds 1.5times the charge starting voltage of the member to be charged.
 9. Theapparatus of claim 8, wherein the oscillating polarity drive voltage isapplied for at least one revolution of the charge roll member.
 10. Theapparatus of claim 1, wherein the oscillating drive voltage during aperiod when the second mode is selected exceeds the drive voltage duringperiods when the first mode is selected.
 11. The apparatus of claim 10,further comprising a resister element coupled between the power supplyand the charge roll member during a period in which the first mode isselected.
 12. The apparatus of claim 1, further comprising a mechanismfor rotating the member to be charged and wherein, upon transition fromthe second mode to the first mode, the mechanism for member rotation isactivated for at least one revolution of the member to be charged. 13.The apparatus of claim 1, further comprising an apparatus for cleaningthe member to be charged wherein said cleaning apparatus is engagedbefore imaging for cleaning from the member to be charged residueextracted from the charge roll member during periods when the secondmode is selected.
 14. The apparatus of claim 1, further comprising animage development apparatus for developing an image on the member to becharged wherein development of images is suppressed during periods whenthe second mode is selected.
 15. An apparatus for applying an electricalto a member to be charged comprising: a power supply for supplying anoscillating voltage signal; a charge roll member situated in proximityto a surface of the member to be charged; and a switch for switchingbetween a plurality of modes wherein: a) in a first mode, an electricalbias is applied from the power supply to the charge roll member, theelectrical bias including a single polarity input drive voltage to saidcharge roll member; and b) in a second mode, an electrical bias isapplied from the power supply to the charge roll member, the electricalbias including an oscillating voltage signal containing multiplepolarity components in order to supply oscillating polarity input drivevoltage to the charge roll member, wherein the second mode is selectedduring a period ranging from 5% to 40% of the time during which themember to be charged is moving.
 16. An apparatus for applying anelectrical charge to a member to be charged, comprising: a power supplyfor supplying an oscillating voltage signal; a charge roll membersituated in proximity to a surface of the member to be charged; and aswitch for switching between a plurality of modes wherein: a) in a firstmode, an electrical bias is applied from the power supply to the chargeroll member, the electrical bias including a single polarity input drivevoltage to said charge roll member; and b) in a second mode, anelectrical bias is applied from the cower supply to the charge rollmember, the electrical bias including an oscillating voltage signalcontaining multiple polarity components in order to supply oscillatingpolarity input drive voltage to the charge roll member, wherein thesecond mode is selected during periods comprising less than 20% of thetime during which the member to be charged is moving.
 17. An apparatusfor applying an electrical charge to a member to be charged, comprising:a power supply for supplying an oscillating voltage signal; a chargeroll member situated in proximity to a surface of the member to becharged; a switch for switching between a plurality of modes wherein: a)in a first mode, an electrical bias is applied from the power supply tothe charge roll member, the electrical bias including a single polarityinput drive voltage to said charge roll member; and b) in a second mode,an electrical bias is applied from the power supply to the charge rollmember, the electrical bias including an oscillating voltage signalcontaining multiple polarity components in order to supply oscillatingpolarity input drive voltage to the charge roll member; and an imagedevelopment apparatus for developing an image on the member to becharged wherein development of images is suppressed during periods whenthe second mode is selected and wherein the development apparatusgenerates an electrical development field having a bias voltage whereinsuppression of development of images is achieved by increasing thedevelopment field bias voltage.
 18. An electrophotographic imagingsystem, comprising: an apparatus for applying an electrical charge to amember to be charged, said charging apparatus, comprising: a powersupply for supplying an oscillating voltage signal; a charge roll membersituated in proximity to a surface of the member to be charged; and aswitch for switching between a plurality of modes wherein: a) in a firstmode, an electrical bias is applied from the power supply to the chargeroll member, the electrical bias including a single polarity input drivevoltage to said charge roll member; and b) in a second made, anelectrical bias is applied from the power supply to the charge rollmember, the electrical bias including an oscillating voltage signalcontaining multiple polarity components in order to supply oscillatingpolarity input drive voltage to the charge roll member, said voltagesignal comprising an oscillating voltage signal that is clipped toremove a selected polarity.
 19. The apparatus of claim 18, wherein theelectrophotographic imaging system undergoes procedures related tosystem warm-up, system shut-down, end of an imaging sequence, and systemidleness for specified periods; and wherein the second mode is selectedfor timed periods triggered by system events selected from the groupconsisting essentially of system warm-up, system shut-down, end of animaging sequence, and system idleness for specified periods.
 20. Theapparatus of claim 18, further comprising an image development apparatusfor developing an image on the member to be charged wherein developmentof images is suppressed during periods when the second mode is selected.21. The apparatus of claim 20, wherein the development apparatusgenerates an electrical development field having a bias voltage whereinsuppression of development of images is achieved by increasing thedevelopment field bias voltage.
 22. A process for applying an electricalcharge to a member to be charged, comprising: a. supplying anoscillating voltage signal from a power supply; b. positioning a chargeroll member proximately to a surface of the member to be charged; and c.selecting between a plurality of modes, wherein: in the first mode, anelectrical bias is applied from the power supply to the charge rollmember, the electrical bias including a single polarity input drivevoltage to said charge roll member, said voltage signal comprising anoscillating voltage signal that is clipped to remove a selectedpolarity; and in a second mode, an electrical bias is applied from thepower supply to the charge roll member, the electrical bias including anoscillating voltage signal containing multiple polarity components inorder to supply oscillating polarity input drive voltage to the chargeroll member.